Chris travels around the emerald band that encircles our planet – the jungles. And he begins by tackling one of the biggest puzzles about the world’s rainforests: why do these places have such a bewildering variety of life, far greater than any other habitat on Earth? His quest begins in Panama where more varieties of birds have been recorded in a single day than anywhere else on the planet. He reveals that the key to the biodiversity of the rainforests is an almost constant climate. This allows insects and everything else that gorges itself on plants, to flourish all year round. So rainforest plants are under constant attack. The result is an unrelenting arms race, in which only the weird survive. Plants with barbs and poisons for defence; mammals, like the three-toed sloth which has evolved a way to digest leaves that nothing else can digest – even if it means taking days to digest them and moving as slow as, well, as a sloth. And then there are the Heliconius butterflies, that have evolved defence mechanisms against the cyanide of poisonous vines – absorbing the poisons into their bodies to use as a defence against their own predators. Chris reveals how evolutionary arms races in jungles have led to a riotous biodiversity of life. Chris travels to Borneo to see how some of the largest animals of the jungle, the orang-utan and the forest elephant are crucial in protecting this biodiversity, by spreading the precious seeds of jungle plants, far from their parent plants, to avoid attack by their predators. He then travels to the Amazon to witness one truly amazing web of relationships centred around the Brazil nut tree. The ecosystem of this tree is too complex for it to be cultivated, so if you’ve ever eaten a Brazil nut, you too, are dependent on its ecosystem. It involves a small rodent, called an agouti, which the tree needs to disperse and plant its nuts, and a rare orchid which attracts a rare male bee, which in turn attracts a female, the o

Chris travels to the savannah of Kenya, the grasslands of Australia and the Cerrado of Brazil to witness how one of our most important ecosystems work – grasslands. The secret of grasslands is not what they have, but what they don’t have – and how they cope. Grasslands are lacking in one crucial nutrient. Nitrogen is the element necessary for all proteins, the building blocks of life. You can’t grow without it, yet nitrogen-poor grasslands around the world support some of the world’s largest animals. Something that’s only possible, thanks to the ways that these ecosystems ‘manage’ their nitrogen. Chris travels to Kenya to see the surprisingly important role that rhinos play in making the grasslands fit for antelope. In the Brazilian Cerrado, he sees how maned wolves get by on a low nitrogen diet by gardening their own fruit. And how anteaters hunt the world’s richest source of nitrogen… not ants, but termites. In Australia, Chris encounters a weird cast of mini grassland characters, such as bandicoots and quols, driven to the edge of extinction by the introduction of alien species. Foxes and domestic cats have removed much of Australia’s natural grassland fauna and, as a consequence, the whole ecosystem has suffered. Finally, Chris returns to East Africa to reveal how one extraordinary ecosystem works – that of the acacia tree. A gecko, a giraffe, an ant and a monkey all depend on this tree for their survival … but what’s really wonderful is how these individuals and the acacia also depend on the actions of each other.

Seasonal forests, such as those that cover the UK and most of North America, are special. Their ecosystems have to cope with drastic change twice a year, in Spring and Autumn. Not only do the inhabitants have to deal with the change, but the entire ecosystem has to transform itself – and it has to do this in perfect synchronicity. In this episode, Chris spends all of his time in the vast seasonal forests of North America. Here, he sees why the whole forest depends on the Autumnal activities of a flying squirrel. And uncovers how the activities of an obscure moth caterpillar in Spring can mean the survival of the Canadian lynx in winter. Perhaps most amazing of all though, is how the great forests of British Columbia depend upon bears to catch salmon and take them into the forest, where their carcasses then fertilise its massive trees. Amazingly, up to 85% of the nutrients that feed these trees come from salmon. That’s how much the tree needs the salmon.

In the final part of the series, Chris makes an epic journey, from the mountain streams of Iceland to the deep of the Indian Ocean, via the wetlands of the Pantanal in Brazil and the coastal swamps of Bangladesh. And on the way, he reveals how our most important ecosystems, overcome the daily challenge of living in medium that is constantly on the move. He shows us how the Pantanal wetland in Brazil, while being a land of the giants (the world’s biggest snake, water lily, rodent, otter – you name it, they all live here), actually couldn’t work without the humble apple snail. Thanks to this animal, the Pantanal is able to recycle nutrients and regenerate itself in all its amazing glory, every year. Chris then travels to the Sunderbans – a vast mangrove swamp at the mouth of the Ganges. After the Pantanal, it seems amazing that anything at all can live in what appears to be a sticky, sulphurous ooze. But, in fact, there is so much life here that this place supports a quarter of the world’s tiger population. Once again, it’s all down to a group of unsung heroes – the crabs on the beach. These animals dig burrows that not only supply oxygen and food to the roots of the mangroves; they also mix the toxic seawater with freshwater so that these trees can thrive. Chris then heads out to sea, to the coral reefs of the Maldives. Here he investigates the puzzle of where reefs get their food. After all, their waters are crystal clear, so where are the nutrients? The answer comes in the very unlikely form of one of the weirdest animals on the planet. Sponges have no eyes or nervous system and can’t move. But what’s really weird is the way they eat their food. They pump seawater through their bodies at such a high rate that they are able to harvest enough nutrients, from the microscopic particles in the water. And, thanks to the sponges capturing these nutrients, the whole reef can thrive. Chris then sails out into the deep ocean to reveal how ocean